The present invention relates generally to systems for measuring optical density and, more particularly, to a novel field usable densitometer for measuring very optically dense materials, such as laser eye protection and optical filters used to protect light-sensitive detectors and instruments.
The use of lasers has become widespread, especially in military equipment on the battlefield. Laser damage can be intentionally inflicted, as from offensively used lasers, or inadvertently, as from lasers used for range finders, targeting, and measuring devices. In military situations, the eyes of personnel and the detectors of sensitive optical instruments need protection from damaging laser radiation. In non-military situations, it is necessary to provide protection from laser radiation to individuals working near or with laser containing equipment. Optical filters are used to block high intensity pulsed or continuous wave laser beams in the visible, near infrared and ultraviolet portions of the spectrum, to prevent eye damage and damage to detectors in the path of the laser beam.
Filter protection is described in terms of optical density, which logarithmically relates transmitted energy to incident energy. If a filter has an optical density of one, it absorbs 90% of the incident energy and transmits 10%; with a density of 2, it would transmit 1/100 of the incident energy. An optical density of 5 means the filter has reduced the power of the laser beam to 1/100,000 of its original power. To be effective, protective filters must have a high optical density at the selected wavelength. Sun glasses normally have an optical density of less than 1 across the entire visible spectrum, but for laser protection, optical densities as high as 10 are desirable at very specific wavelengths.
For purposes of measuring optical densities, it is well known to employ an instrument known as a double beam spectrophotometer. A double-beam spectrophotometer typically includes a light source, a monochromator, a beam splitter, a sample detector, and a reference detector. These components, together with a standard power supply, normally occupy a volume of at least one cubic foot. The light source provides a beam of light, which is split by the beam splitter into two beams. One of the light beams is directed through a sample before or after passing through the monochromator to the sample detector. The monochromator divides a polychromatic beam into a nearly monochromatic beam. The other beam passes unimpeded to the reference detector. The intensities of the light beams measured by the detectors are compared, the ratio being indicative of the optical density of the sample. Typically, the instrument is calibrated against a sample of known optical density and, thereafter, unknown samples are examined.
There are various possibilities for the choice of the main components of spectrophotometers. The light source usually comprises a continuous-emission lamp, such as halogen, deuterium, tungsten or xenon. Prism or grating dispersion devices are used for the monochromator. The detectors are generally photomultipliers, phototubes or silicon photodiodiodes. The beam splitter is typically a flat quartz plate or a partially reflecting mirror. The various kinds of available components and the various possible structures can be combined in numerous ways to construct a double beam spectrophotometer.
A low optical density, up to about 4, can be measured directly on a double beam spectrophotometer for wavelengths within its operating range. However, to ensure eye safety, many commercial laser eye protectors have very high optical densities at some laser wavelengths. As indicated previously, optical densities in the range of 8 to 10 are not uncommon for protective optical filters. Such high attenuations are beyond the capabilities of the double beam spectrophotometer to measure. Instead, inaccurate extrapolations of spectrophotometer transmission measurements must be made.
As indicated previously, spectrophotometric measurements give the optical density of the filter or other material to a narrow-band, low-power, continuous wave light source, such as halogen, deuterium, tungsten or xenon. The surest way to confirm the protection capability of a laser-protective filter, however, is to expose the filter directly to the laser which is desired to be protected against. Moreover, the advantageous characteristics of laser light, in particular its high spectral brightness and purity, facilitate optical density measurements of very optically dense samples. U.S. Pat. No. 4,068,956 to Taboada discloses a densitometer system which uses a pulsed dye laser for measuring highly dense samples. This system, however, is optically complex, requiring multiple components including eight lenses, one beam splitter, two mirrors, multiple filters, one wedge, one photodiode or photomultiplier and one oscilloscope. Like the double beam spectrophotometer, the Taboada system includes the light source as part of the system. As a result, it is quite costly and very large, requiring a 5 m optical bench. Undisclosed special instrumentation is required in order for the disclosed apparatus to provide adequate measurement of high energy, ultrashort pulses. The Taboada system requires independent calibration through the use of standardized filters of known optical density. In addition, the optical density filters used in this system can be subject to "bleaching" and loss of optical density properties with such ultrashort pulses. Finally, it should be noted that, to be suitable as a protector against laser radiation, a filter must maintain its optical density under exposure, not only to the high peak power densities associated with pulsed lasers, but also to the continuous wave power densities capable of eventually destroying the filter material.
The need therefore exists for a portable, simple, cost-effective densitometer which can measure optical densities to 10 and which is compatible with laser light sources of any pulse duration, continuous wave through ultrashort.